Add README gap analysis against Flutter health to clarify Maui.Health roadmap

[Copilot]

This issue asked for a comparison of Maui.Health vs pub.dev/packages/health (carp-health-flutter) to identify missing capabilities and improvement opportunities. This PR adds a focused, actionable gap analysis to the README so contributors can align future work with proven patterns from the Flutter ecosystem.

• What was added

  • New README section: “Comparison with Flutter health plugin (gap analysis)”
  • Prioritized improvement areas:
    1. Permission lifecycle parity (revocation-status checks + settings guidance, background-read authorization APIs)
    2. Workout route support (start/append/finish/discard route APIs)
    3. Bulk delete by metric type + time range
    4. Broader cross-platform metric DTO coverage
    5. Query-time recording-method filters (manual vs automatic)

• Clarity updates

  • Adjusted wording to avoid implying Android permissions can be programmatically revoked.
  • Expanded first use of heart rate variability (HRV) for readability.

## Comparison with Flutter `health` plugin (gap analysis)

After reviewing `pub.dev/packages/health` and `carp-dk/carp-health-flutter`,
these are the most relevant improvements for `Maui.Health`:
1. Permission lifecycle parity
2. Workout route support
3. Bulk delete by type + time range
4. Broader metric coverage
5. Query filters for manual vs automatic entries